An example of a "default singleton"

Foo.php

<?php

class Foo
{
    private $name;
    private static $defaultInstance = null;

    /**
     * Public constructor: create as many instance as you want
     */
    public function __construct($name)
    {
        $this->name = $name;
    }

    public static function getDefaultInstance()
    {
        if (null === self::$defaultInstance) {
            self::$defaultInstance = new self('default');
        }

        return self::$defaultInstance;
    }
}